Signal amplifying system



Feb. 26, E. Q KElzER SIGNAL AMPLIFYING SYSTEM v Filed oct.. 2, 195o Patented Feb. 26, 1952 SIGNAL AMPLIFYING SYSTEM Eugene 0. Keizer, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 2, 1950, Serial No. 187,933

Claims. (Cl. Z50-27) The present invention relates to improvements in signal amplifying systems and more particularly although not necessarily exclusively to improvements in the neutralization of cascade electron discharge tube amplifiers.

More directly, the present invention is involved with the provision of a novel neutralizing circuit arrangement for cascaded triode amplifiers and particularly where these amplifiers are to be simultaneously gated or switched on and off by auxiliary control signals.

In many types of electrical equipment it becomes expedient to use a sampler type of amplifying circuit such as the gated amplifier or electronic switch. Such an amplifier circuit is generally provided with two input terminals, one adapted to receive intelligence signals and the other adapted to receive a control signal. It is the function of the control signal to turn the amplifier on and ofi so that it may pass only selected portions of the intelligence signals. In order to obtain more perfect gating action use is sometimes made of two or more gating amplifying stages connected in cascade, the gating or control pulse being applied to all stages simultaneously to turn them -on or off as may be required while the intelligence signal is clrcuited in normal cascade amplifier fashion. Especially where triode amplifiers are used, this cascade system of signal amplification presents a number of problems. The first problem is that of capa citive loading of the gating signal source so that a steep wave front gating signal is diilicult to realize; secondly, the triode amplifiers must necessarily be neutralized and, as will be later seen. such neutralization has not heretofore been easy to obtain in view of the common input circuit to all of the amplifiers from the gating control signal source. Numerous undesired feedback loops present themselves in the classical approach to the neutralization of the individual amplifiers in such a circuit. Perhaps no greater need exists for a simple and effective gated amplifier than in present-day systems of dot multiplex color television receiver synchronization. A thorough discussion of the use and place of the gated amplifier in such a color television receiving system is presented in a pamphlet entitled Recent Developments in Color Synchronization in the RCA Color Television System by the RCA Laboratories Division of the Radio Corporation of America and dated February 1950. It is there described that the horizontal synchronizing vision signal lwill 'have situated on their back porches a burst of synchronizing frequency which, by way of example, may be in the neighborhood of 3.6 megacycles. Gating means are then provided in .the receiver for sampling or extracting from the incoming signal the bursts of synchronizing component.

The separated burst is then fed toa frequency comparison circuit which maintains a local oscillator in the receiver at the proper frequency and phase for synchronously conducting well known dot multiplex demodulation of the incoming signal.

Experimentation with gating circuit suitable for use in extracting the burst component from the television signal has led to the conclusion that it is highly desirable to use a plurality .of amplifier-stages which are connected in.cascade in so far as the received televisionsignalis concerned but are simultaneously turned .on and off or4 gated by a-control pulse which is so related in time to the back porch of the horizontal synchronizing pulse that only 3.6 megacycle burst component appears at the output of the cascade amplifier. If properly neutralized triode .ampliflers are used, for the amplifier stages the above mentioned problems with respect to capan citive loading of Vthe gating signal source-as well as the presence of unoc'irable feed back loops will become of primary concern.

It is thereforean object of the present inventionto providea neutralizing circuit arrangement for -'cascaded triode amplifiers which will minimize the eiiects ofcapacitive loading on terminals connected with the input electrodes of any of the individual stages of the amplifier.

It is another pirpose ofthe present invention to provide a new and useful signal gating circuit of the cascade variety described above which presents a minimum of undesirable feedback paths.

'Itis another purpose of the present invention toprovide an improved signal gating circuit for separating the burst component from incoming dot multiplex signal color television signals.

In the realization of the above objects and features of advantage the present invention contemplates the use of first and second triode discharge tubes having respective load and polarizing circuits connected between their anodes and cathodes. A coupling capacitor is'then-provided 'between'the anode -of 'the first triode and V.the

control electrode of thesecond triode. Phasereversing means are then connected "betweenthe anode an'd controlelectrode of the-secondtxicde 'topro'duce'a degree fof neutralization whilemeutralization of the first triode is accomplished through th` connection of a resonant circuit between the control electrodes of the 'first and second triodes. For color television burst separation, a tunedcircuit may be coupled with the anode circuits of each of the triodes. The color television signals are then applied directly to the control 'electrode or grid of the first triode while gating signals synchronized with the arrival of the burst are applied through a resistive isolating network to both of the control electrodes of the first and second triodes.

A better understanding of the present invention as well as an appreciation of its other objects and features of advantage may be obtained from a.reading of the following description especially when considered in connection with the accompanyingsingle figure of the drawing in which is shown a combination block and schematic diagram of a dot multiplex color television receiving system in which the present invention finds ready application.

Turning now to the single figure. there is indicated at l some form of television signal receiver suitable for the reception and demodulation of dot .multiplex color television signal having a burst synchronizing component. Suitable circuitry for this purpose is illustrated and described in another pamphlet entitled A Simplified Recever for the RCA Color Television System dated February, 1950 and written by the RCA Laboratories Division of the Radio Corporation of America. The output of the receiver is, as further described in the latter referenced pamphlet. coupled to the input of three sampler circuits I2, 4 and I8, respectively designated as the red. blue and green samplers. These samplers are nothing more than special forms of gated amplifiers with which the present invention is not necessarily concerned but may be applied thereto. Suitable circuitry for these samplers is described in the latter referred to pamphlet.

Bv way of example, a 3.58 megacycle oscillator at |8 is indicated as a source of gating or sampling signal for the sampler circuits l2, I4 and I8. The 120 degree phase shifting network 20 and 22 provides the necessary staggering of the sampling conducted by the samplers. 'Ihe outputs of the Samplers l2, |4 and i8 are respectively set to corresponding video amplifiers 24, 26 and '28. 'Ihe output of the amplifiers are, in turn,

connected to the cathodes 30, 32 and 34 of a threegun color kinescope 36 also described in the latter referred to article. A by-passing circuit 38 is also indicated in the figure for by-passing a desired color television signal component around the sampling stages and` applying them to the grids 40 of the color kinescope 38.

The output of the television receiver i0 is also applied to a conventional sync separator circuit 42 which separates vertical and horizontal synchronizing signal infomation from the incoming color television signal. A suitable circuit arrangement for this function is disclosed by way of U. S. Patent Ser.' No. 2,207,775 by A.'V. Bedford, July 18, 1940. The separated vertical sync pulses appearing at terminal 44 of the sync separator are conventionally applied to the vertical deflection circuit 48 while the horizontal syn- Achronizing pulses appearing at terminal 48 of the sync separator are applied to the horizontal deflection circuit 50. Suitable vertical and horizontalA deflection circuitsfcr this purpose are shownf in an article entitled Television R45-- ceivers" by Antony R. Wright appearing in the March 1947 issue of the RCA Review. As described 'n the above RCA Review article, there appears at the output of the deflection circuit a horizontal iiy-back pulse which by way of example has been illustrated in the drawings at 52 and is corfidered as extending in the negative going direction.

The horizontal fly-back pulse 52 is then utilized to synchronize a square wave generating circuit based upon the discharge tubes 54 and 56. The square wave circuit is of Eccles-Jordan type as described in detail on pages 171 through 176 of a book entiled Ultra High Frequency Techniques" by Brainard, Koehler, Reich, and Woodruff and published by D. Van Nostrand Co., Inc., in 1942.` Such a square wave generating-circuit is common-place and well-known in the art and since it forms no part of the present invention a detailed description of its operation will be omitted. It is well-known that .the right-hand triode 56 of the circuit is periodically rendered conducting and non-conducting to forml the square wave 58. This square wave 58 is thereby used by the present invention asta keying pulse for the gated amplifier about to be described. It is noted that due to the connection of the anode of triode 56 to ground potential through resistor 60 that the peak 62 of the saw-tooth wave 58 goes no higher than ground potential or zero volts. This peak voltage represents non-conduction in the triode 56. The negativegoing extremity 84 of the square wave 58 of course represents maximum conduction in the discharge tube 58 and represents some D. C. potential which is negative with respect to ground.

As described in the above referenced pamphlet entitled A Simplified Receiver for the RCA Color Television System it is necessary to control the 3.58 mc. oscillator i8 in accordance with the burst component of the received color television signal. As stated above, this burst oo mponent appears on the back porch of the horizontal synchronizing pulse. This arrangement is shown in the figure at 68. The horizontal sync pulse 68 has a 3.58 mc. burst component at 10 situated on the back porch of the horizontal sync. As further described by the last referred to pamphlet, it is necessary to separate the burst component 10 from the received color television signal. This is accomplished by means of the tried.. discharge tube shown at l2.

In accordance with the present invention the color television signal appearing at the output of the receiver |0 is coupled by `apacitor 14 to the grid 18 of the left-hand triode section 18. The cathodes of both triode sections are groundcd. The anode of the triode section 18 is supplied with polarizing potential through resistors 82 and 8l from the positive B power supply terminal at at. Voltage variations on the anode 80 are coupled via capacitor B8 to the control electrode or grid of right-hand triode section 82. Positive polarizing potential for the anode 94 of the triode 92 is supplied through the upper section 96 of inductance 98 whose tap |00 is connected with the right-hand terminal of resistor 82. The voltage appearing across the second section |02 of the inductance 88 will of course be in phase with the voltage appearing across the upper section 98 so that with respect to the terminal |00 a push-pull signal appears across the inductance. Terminal |00 is effectively at A. C. ground potential by merit of the capacitor |08. To accentuate the response of the gated stageto the desired burst frequency; a tuned circuit |08 is connected in shunt with theanode circuit of-'the first triode 18. The output circuit of triode 12 is similarly made resonant at thefirst frequency by means of a variable capacitor il 0. The grid 18 of the triode 18 is connected' with ground through resistors ||2 and i |4 while the grid of triode 92 is connected with ground through resistor ||6.

Irv-further accordance with the present invention, the gating pulse 58 for control of the gated amplifier is applied at the junction of the resistances ||8 and |20. A capacitor |22 shunts a portion of the resistor 8. The exact manner in which the gating pulse 58 is applied to the resistors ||8 and |20, the connection of the capacitor |22, and the bridging of resistor ||8 and |20 across the upper terminal of ||4 and IIB form the subject matter of my copending U. S. patent application Ser. No. 187,934 filed concomitantly herewith and entitled "Gated Ampliiler Circuits." Sufiice it to say for the purpose of the present invention that positive going gating pulses appear at the resistors ||4 and ||B and that the amplitude of these pulses is sumcient to overcome the negative cutoff bias imposed on the grids of the triodes 18 and 92 which is present under dynamic operating conditions.

The operation of the circuit thus far described is substantially as follows. The positive going keying pulse 58 applied to the grids of the triodes 18 and 92 causes grid current conduction in these triodes. This grid current conduction causes a negative grid current bias to be developed across the resistors ||2, i|4 Aand H6. This bias supplements the average negative value of the multivibrator anode so that the grids 18 and 83 of the triodes 18 and 82 are swung to plate current cutoil during the negative going pulse 64 of the keying pulse l58. This means that the triodes 18 and 92 will act to amplify received color signal only during the positive-going portion of 'the keying pulse 50. Since the keying pulse 58 occurs during the back porch of the horizontal pulse 88 demodulated by the receiver, it follows that the triode 12 will be on only during the burst interval of the received signal.

Consequently, there will appear at the anode 80 of the triode 18 a wave form substantially the same as that indicated at |24. Due to the rather high Q of the circuit |08 it is apparent that it takes some time for the 3.58 sinusoidal component to develop. Correspondingly, it takes some time after the occurrence of the keying pulse 58 before the sinusoidal variation to the tank circuit |08 has completely died out. The triode 82 in turn samples the output of the triode 18 so as to give an even cleaner burst wave form indicated. by way ci example, at |28. As described above. since the signal appearing across the lnductance 88 is a push-pull signal with respect to'ground the burst component |28 is applied to theudouble diode |28 in typical frequency comparison fashion.. The output of the oscillator i8 is of course fed to the center tap |80 of resistor |32 connected between the anode of one and the cathode of the other of the two diodes. The diodes are isolated from the DC appearing on the lnductance 88 by means of capacitors 84 and |38. The cathode of the one and the anode of the other of the diodes are connected together and also to the low pass illter formed by resistor |88, capacitor and resistor |42. As described in the above referenced pamphlet A Simplified Receiver for the RCA Color Television System,"

the D. C. voltage appearing at the right-hand terminal of resistor |42 may be used to control the=reactancetube |44 which-.in tum adjusts the frequency theaoscillator |8 so that it correspends with e frequency of the received burst.

The fact that discharge tubes 18 and 92 of thegating circuits are triodes requires that some means be provided by accomplishing neutralization. This, in accordance with the present invention is accomplished by means of the capacitor |46 which feeds back voltage onto the grid 83- of thetriode 92 which is in phase oppositionA with the unwanted feed-through voltage appearing on the anode 94 of the same triode' dueV to grid plate capacity. This phase relationship-exists, of course, because of the previously described push-pull voltage conditions existing across the lnductance 98. Neutralization of the first triode 1R is. accomplished through the use of lnductance |48, resistor |50 and ccpacitor |52. This series combination feeds back out of phase voltage on the grid 15 of the triode 18 and thus accomplishes neutralization. The neutralization of the triode 92 will be seenA to be the well-known Rice "plate-side type of neutralization while the neutralization of the first triode 18 is substantially `the same as wellknown-coil neutralization variety.

The particular choice and circuit arrangement of the neutralization provided by the present invention is extremely important to the successful operation of the gated amplier stages. By using the particular arrangement of the present invention the circuit capacitor imposed across the output of the multivibrator 58 is kept at a minimum while undesirable additional feedback from the output to the input of the rst amplifier is avoided.

In practice the first triode may be thought of as being inductively neutralized. That is, an inductive path is shunted from the anode to grid ofthe triode. The value of this lnductance which is shown at |48 is equal in magnitude to the capacitancereactance of the anode grid path of the tube plus stray capacity thus forming a high impedance and resonant circuit which reduces the feed-through of the tube. The value of inductance |48 is therefore such that its reactance at 3.58 mc. is greater than the reactance of capacitors |52 and 88 at the frequency. The path providedby the lnductance |48, resistance |50 A and capacitor |52 then becomes the means for applying gating voltage for the second triode grid. The impossibility of maintaining as low a capacitive loading on the multivibrator 56 and providing a minimum of undesirable additional feed-back between the triodes through the use of other combinations of neutralization methods clearly points out the novelty and value of the present invention.

It will be understood, of course, that the purpose of resistor |50 is to damp the transient response which might otherwise be generated in the series circuit of the lnductance |48 and capacltor |52. Furthermore. it will be apparent that the particular manner of applying neutralization to the two cascade amplifier stages andv the-technique of applying signals to these stages disclosed hereinabove is applicable to other circuit arrangements than merely gate-type amplifiers. 'Ihe present invention is useful in any circuit where in-phase voltages are to be applied to the grids of successive cascade amplifiers while a second and different signal voltage is to be applied to the input only of the cascade amplifier.

Having thus described my invention what is claimed is:

1. In a gated amplifier for electrical circuits, the combination of, a terminal adapted to receive a gating signal of desired duration, an intelligence signal terminal adapted to receive an electrical intelligence signal which is to be gated by said gating signal, a first and a second electron discharge tube each having at least an anode, cathode and control electrode, a respective load and polarizing circuit connected between the anode and cathode of each of said first and second discharge tubes, a capacitor connected between said first discharge tube anode and said second discharge tube control electrode, means connected with said second discharge tube load circuit and said second discharge tube control electrode for feeding onto the second tube control electrode voltages which are opposite in phase to voltages appearing on said second tube anode, an inductance and a capacitance connected in series combination between the con trol electrodes of said rst and second discharge tubes, impedance means connected with said gate signal terminal and said rst and second tube control electrodes for concomitantly applying thereto the gating signal, and means for coupling said first tube control electrode to said intelligence signal terminal.

2. Apparatus according to claim 1 wherein said intelligence signal includes a periodically recurrent index component, and wherein the gating signal for said gating signal terminal is timed to occur during said index component, and wherein the load circuit for said second discharge tube includes an inductance-capacitance resonant circuit resonant to a frequency value which is integrally related to the recurrence frequency of said index component.

3. Apparatus according to claim 1 wherein said intelligence signal includes a periodically recurrent index component, and wherein the gating signal for said gating signal terminal is timed to occur during said index component. and wherein said series combination inductance and capacitance is made resonant to a frequency integrally related to the recurrence frequency of said index component.

4. Apparatus according to claim 1 wherein said opposite phase voltage feeding means includes a timed circuit resonant to the frequency of said index component.

5. In a signal amplifying system. a first and second electron discharge tube each having at least an anode. cathode and control electrode, a respective load and polarizing circuit connected between the anode and cathode of each of said first and second discharge tubes, a capacitor connected between said first discharge tube anode and said second discharge tube control electrode, means connected with said second discharge tube load circuit and said second discharge tube control electrode for feeding onto the second tube control electrode voltages which are opposite in phase to voltages appearing on said second tube anode, an inductance and a capacitance connected in series combintion between the control electrodes of said first and second discharge tubes, and a single input circuit connected between the control electrodes and cathodes of said discharge tubes.

6. Apparatus according to claim 5 wherein said opposite phase voltage feeding means comprises an inductance and capacitance connected in parallel. a part of said inductance being connected in series with said second tube anodecathode circuit with means for applying tne voltages appearing across the remainder of the inductance to said first discharge tube control electrode.

7. In an electrical circuit, a first and second electron discharge tube each having at least an anode, cathode and control electrode, a polarizing potential power supply termial, a datum terminal forming a reference for said power supply terminal, means connecting each of said first and second tube cathodes with said datum terminal, a first resistance connected from said first tube anode to said power supply terminal, a first parallel tuned circuit capacitively coupled to said first tube anode, a tapped inductance having a first and second section defined by the tap and the inductance extremities, connections placing said inductance first section in series with a circuit from said second tube anode and said power supply terminal, a capacitor connected in shunt with said inductance to forni a second parallel tuned circuit, a vcapacitor connected from said inductance second section to said second tube control electrode, a second inductance and another capacitance connected in series between the control electrodes of said first and second tubes, and grid return resistances connecting said first and second tube control electrodes with said datum terminal.

8. Apparatus according to claim 7 wherein there is additionally provided an isolating resistance which is connected between said first tube control electrode and said grid return resistances, and wherein there is also provided a rst signal input circuit connected with said grid return resistances and a second signal input connected on the control electrodes side of said isolating resistance.

9. Apparatus according to claim 8 wherein there is additionally provided means for biasing said first tube into static anode current cutoff. and wherein said first input circuit is designated to receive positive going keying pu'lses of suilcient amplitude to produce momentary conducvtion in said first tube 4while said second circuit is REFERENCES CITED The following references are of rcord in the file of this patent:

UNITED STATES PATENTS Name Date Lester et al Jan. 9, i

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